Answer
$(-\infty,0)\cup(0,3)$
Work Step by Step
First I graph the function.
Then, according to the graph of the function: $f(x)<0$ if $(-\infty,0)\cup(0,3)$ because the graph of the function is under the $x$-axis ($y=0$), which means that the value of the function is less than zero on these intervals.
$0$ and $3$ are not included because the inequality involves strictly less than.